1. Field of the Invention
This invention relates to tree retardant additives. More particularly, it relates to a combination of tree retardant additives which provides a significantly high level of both water tree and electrical tree retardation when added to polymeric electrical insulation. This invention especially relates to a combination of tree retardant additives which acts synergistically when admixed with a polymeric electrical insulation.
2. Description of the Prior Art
Polymeric compositions are well-known and are used extensively as insulation materials for wire and cable. As an insulator, it is important that the composition have various physical and electrical properties, such as resistance to mechanical cut-through, stress crack resistance and dielectric failure. Recent publications have indicated that water tree growth and electrical tree growth in the insulation are particularly important problems since they are associated with, though not necessarily totally responsible for, dielectric failure. The term "tree" has been applied to this type of insulation breakdown since the area of failure is a void space having the appearance of a tree in profile, i.e., the shape of a tree trunk and its upper foliage. Treeing usually is a slowly developing phenomenon and may take years to cause a failure in the insulation.
An important application for an insulation material is in high voltage transmission and distribution cable, especially in direct buried underground service. Two types of trees have been observed in these power cables, to wit, electrical trees and water trees, which are sometimes referred to as electrochemical trees. It is generally believed that electrical trees are generated by corona discharges causing fusion and breakdown of the polymer, whereas water trees are usually observed in cables buried in wet locations and have a different appearance compared to electrical trees. Metal ions are usually found in water trees.
U.S. Pat. No. 4,144,202 of Ashcraft et al. relates to inhibiting the electrical breakdown of insulation by water treeing in dielectric materials based on ethylene polymers. As disclosed in this patent, water treeing is inhibited in the ethylene polymer compositions by employing therein organo silane compounds having an epoxy containing radical. German Offenlegungsschrift No. 2,737,430 discloses that certain alkoxysilanes added to polyolefin insulation prevent water-tree formation.
U.S. Pat. No. 4,206,260 of McMahon relates to insulation particularly suitable for high voltage power cable containing an effective amount of an alcohol of 6 to 24 carbon atoms which imparts electrical tree growth resistance to the composition.
U.S. Pat. No. 3,553,348 of Betts, and British Pat. Nos. 1,248,256 and 1,277,378 of General Electric Company relate to mineral filled polymer compositions useful as electrical wire and cable insulation. The mineral filler comprises at least 25% of the composition and is treated with an organosilane such as an alkyl alkoxysilane or a vinyl alkoxysilane to decrease the porosity of the composition. The silane interacts with the reinforcing filler and apparently facilitates a filler-polymer interaction. None of these patents teach or suggest that addition of an organosilane to an unfilled polymer composition will beneficially enhance both the water treeing and electrical treeing resistance of the polymer composition.
Commonly assigned U.S. Pat. Application Ser. No. 161,932, filed June 23, 1980 and now U.S. Pat. No. 4,299,713 discloses a number of organic compounds described by a generic formula, which provides both electrical tree and water tree retardant properties to an unfilled polymeric composition, i.e. a polymer composition containing 0 to 10% mineral filler. A group of preferred hydrocarboxyalkoxy silanes are disclosed in this patent application which are described by the formula: ##STR1## wherein
R.sub.1, R.sub.2 and R.sub.3 are the same or different and are O C.sub.n H.sub.2n OR.sub.4, C.sub.1 to C.sub.8 alkyl, C.sub.1 to C.sub.8 acyloxy, C.sub.2 to C.sub.8 alkenyl, C.sub.6 to C.sub.18 aryl or substituted aryl or C.sub.6 to C.sub.18 aryloxy or substituted aryloxy,
R.sub.4 is C.sub.1 to C.sub.8 alkyl or C.sub.6 to C.sub.18 aryl or substituted aryl, and
n is 1 to 8.
Vinyl-tris (2-methoxyethoxy) silane is particularly preferred among these silanes. The preferred silanes of this application exhibit both water tree resistance and electrical tree resistance when present at a concentration of about 1.5% in the polymeric composition. However, the electrical insulating properties of these compositions, as measured by dielectric constant and dissipation factors, are in some instances, somewhat reduced from those exhibited by the polymeric component of the composition.
It is an object of this invention to provide a polymeric composition having good water tree resistance and good electrical tree resistance while simultaneously exhibiting satisfactory electrical insulating properties as measured by dielectric constant and dissipation factor.
It is another object of this invention to improve the electrical insulating properties of polymeric compositions containing water tree and electrical tree retardant additives.
It is a further object of this invention to improve the electrical insulating properties of the hydrocarboxyalkoxy silane-containing polymeric compositions of U.S. Pat. No. 4,299,713